The dreams of Mary Shelley, author of Frankenstein, involved a pale student kneeling beside a corpse that was jerking back to life. Paul McCartney's contained the melody of Yesterday, while director James Cameron's inspired the Terminator films.
With their eerie mixture of the familiar and the bizarre, it is easy to look for meaning in these nightly wanderings. But why do our brains take these journeys, and why do they contain such outlandish twists and turns?
Unfortunately for armchair psychoanalysts, Sigmund Freud's attempts to interpret dreams remain hotly disputed. Nevertheless, neuroscientists and psychologists have recently made big strides in understanding the way the brain builds our dreams and the factors that shape those curious stories. Along the way, they have found startling hints that our use of technology may be permanently changing the nature of this fundamental human experience.
Anyone who has ever awakened feeling amazed by a dream, only to forget its contents before reaching the shower will understand the difficulties of studying such an ephemeral state of mind. Some of the best attempts to catalogue dream features either asked participants to jot them down as soon as they woke up or had volunteers sleep in a lab where they were awakened and immediately questioned at intervals in the night.
Such experiments have shown our dreams tend to be silent movies: Just half contain traces of sounds. It is even more unusual to enjoy a meal or feel damp grass beneath your feet while asleep: Taste, smell and touch appear only very rarely in dreams.
Similar studies have tried to pin down factors that might influence what we dream about, but with little effect.
More recently, scientists have been looking at the brain's activity during sleep for clues to the making of dreams. Of particular interest is the idea that sleep helps to cement our memories for future recall. After first recording an event in the hippocampus -- which can be thought of as the human memory's printing press -- the brain transfers its contents to the cortex, where it files the recollection for long-term storage.
This has led some psychologists to suspect certain elements of a given memory may surface in our dreams as the different pieces of information are passed across the brain. Studying participants' diaries of real-life events and comparing them with their dream records, Mark Blagrove and his team from Britain's Swansea University have found memories enter our dreams in two separate stages. They first float into our consciousness on the night after the event itself, which might reflect the initial recording of the memory; then they reappear five to seven days later, which may be a sign of consolidation.
Even so, it is quite rare for a single event to appear in a dream in its entirety; instead, memories emerge piecemeal. "What usually happens is that small fragments are recombined into the ongoing story of the dream," says neuroscientist Patrick McNamara at Northcentral University in Prescott Valley, Ariz. And the order in which the different elements appear might reflect the way a memory is broken down and then repackaged during consolidation.
One of McNamara's studies, which compared an individual's dream and real-life diaries over a two-month period, found a sense of place -- a recognizable room, for instance -- was the first fragment of a memory to burst onto the person's dreamscape, followed by characters, actions and finally physical objects.
The sleeping brain also allows us to see associations between different events in life. This might dredge up old memories and plant them in our dreams, which in turn might explain why we often dream of people and places that we haven't seen or visited for months, or even years.
Our dreams are more than a collection of characters and objects, though. They come in many different styles -- from a trivial and disordered sequence to an intense poetic vision -- and our emotional undercurrents seem to be a guiding force. Ernest Hartmann, a professor of psychiatry at Tufts University in Medford, Mass., has studied the dream diaries of people who had recently suffered a painful personal experience or grief. He found they were more likely to have particularly vivid dreams that focus on a single central image rather than a meandering narrative. These dreams also were more memorable than others from more placid times.
Perhaps the intense images are an indication of how difficult it is to integrate a traumatic event with the rest of our autobiography, Hartmann says. "I think it makes a new trauma less traumatic," Hartmann says, though he acknowledges his hypothesis is difficult to prove.
Despite these advances, mysteries remain. Top of the list is the question of the purpose of dreams: Are they essential for preservation of our memories, for instance, or could we store our life's events without them? "There's no consensus," McNamara says. But if we can understand their origins, he says, we would get a better grasp on consciousness in general.
Then there's the impact of technology. Some research suggests that as TV shifted from black and white to colour, it may have caused a similar shift in dreams. Eva Murzyn at the Britain's University of Derby has found people who take part in the World of Warcraft online role-playing game incorporate its user interface into their dream adventures.
Jayne Gackenbach at Grant MacEwan University in Edmonton has found online gamers report a greater sense of control over their dreams, with the feeling they are active participants inside a virtual reality. She points out gamers are more likely to try to fight back when they dream of being pursued by an enemy, for instance. Ironically, this interaction seems to make their dreams less scary and more exciting. "They say things like 'This was a nightmare, but it was awesome.' They are invigorated by it," Gackenbach says.
If you're after a more peaceful night, you might want to take inspiration from Hervey de Saint-Denys, an early dream researcher in the 19th century, who found certain scents could direct his dreams. To prevent his own expectations from clouding the results, he asked his servant to sprinkle a few drops of perfume on his pillow on random nights as he slept. Sure enough, he found this led his dreams to events associated with that particular scent. More generally, recent studies confirm sweet smells can spark emotionally positive dreams.
Robson is a features editor at New Scientist.